Circuit For Packaged Roof Top Units

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hbendillo

Senior Member
Location
South carolina
When I get the spec sheets on mechanical units I need to circuit, the specs usually show Minimum Circuit Amps (MCA) and Maximum Overcurrent Protection (MOP). I always do a calc using the raw data to make sure the MCA and MOP look correct. The following are the specs on a packaged unit I received today.

Compressor 1 RLA - 23 amps
Compressor 2 RLA - 27.5 amps
Supply Fan FLA - 18.9 amps
Condenser Fans FLA - 10.5 Amps (3 x 3.5)
Heater FLA - 86.6 amps

Specs give MCA of 111.47 and MOP of 125 amps.

This heat and cooling will not run concurrently with this unit. So, largest running load will be heater and supply fan.

My MCA would be: 86.6 A x 1.25 + 18.9 amps= 127.15 amps. Breaker would be 130 amps.

The only way their numbers work would be to treat the heat load as a non-continuous load. I could not find any support for considering the heat in this case as a non-continuous load. Searched around on this site an other resources and found nothing to support it as a non-continuous load.

Am I missing something?
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Did they give you the KW??
If you assume they have already figured the heat at 125% to come up with the 86.6, add the supply motor at 1.25 and a bit for controls and you come real close to their number.
 
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hbendillo

Senior Member
Location
South carolina
augie47 said:
Did they give you the KW??
If you assume they have already figured the heat at 125% to come up with the 86.6, add the supply motor at 1.25 and a bit for controls and you come real close to their number.
Click to expand...

Looked further. The heat is listed at 72 KW on one of the other spec sheets which comes out to 86.6 amps at 460/3PH. So they apparently didn't add the 1.25 for heat load. I come close to the number when I take the supply motor at 1.25 and add the heat load a 1.00. I just don't think you can do that.
 
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Npstewart

Senior Member
I have never been able to match the HVAC nameplate loading to the MCA and I have been trying for years. The only thing I can think of is they have safety factors built in somewhere based on how loaded the compressors are.
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I can only agree that it makes no sense other than them figuring the load at 1.0
 
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